A little bit different solution to this project

var inputText = document.getElementById("new-task");
var addButton = document.getElementById("add-button");
var incompleteTasks = document.getElementById("incomplete-tasks");
var completedTasks = document.getElementById("completed-tasks");

var crossTask = function () {
    var listItem = this.parentNode;
    completedTasks.appendChild(listItem);
    this.onchange = uncrossTask;
}

var uncrossTask = function () {
     var listItem = this.parentNode;
     incompleteTasks.appendChild(listItem);
     this.onchange = crossTask;
}

var editTask = function () {
     var listItem = this.parentNode;
     var label = listItem.querySelector("label");
     var editInput = listItem.querySelector("input[type=text]");

     if (listItem.classList.contains("editMode")) {
        var value = editInput.value;
        label.innerText = value;
     } 

     else
        editInput.value = label.innerText;
     listItem.classList.toggle("editMode");
}

var deleteTask = function () {
    var listItem = this.parentNode;
    var ul = listItem.parentNode;
    ul.removeChild(listItem);
}


var bindEvents = function (listItem, checkboxFunction) {
     var checkBox = listItem.querySelector("input[type=checkbox]");
     var editButton = listItem.querySelector("button.edit");
     var deleteButton = listItem.querySelector("button.delete");

     checkBox.onchange = checkboxFunction;
     editButton.onclick = editTask;
     deleteButton.onclick = deleteTask;
}

for (var i = 0; i < incompleteTasks.children.length; i++) {
    bindEvents(incompleteTasks.children[i], crossTask);
}

for (var i = 0; i < completedTasks.children.length; i++) {
    bindEvents(completedTasks.children[i], uncrossTask);
}


addButton.addEventListener("click", function () {
     if (inputText.value!="") {
        var newListItem = document.createElement("li");

         var checkBox = document.createElement("input");
         checkBox.type = "checkbox";

         var label = document.createElement("label");
         label.textContent = inputText.value;

         var inputField = document.createElement("input");
         inputField.type = "text";
         inputField.value = inputText.value;

         var editButton = document.createElement("button");
         editButton.textContent = "Edit";
         editButton.classList.add("edit");

         var deleteButton = document.createElement("button");
         deleteButton.textContent = "Delete";
         deleteButton.classList.add("delete");

         newListItem.appendChild(checkBox);
         newListItem.appendChild(label);
         newListItem.appendChild(inputField);
         newListItem.appendChild(editButton);
         newListItem.appendChild(deleteButton);

         incompleteTasks.appendChild(newListItem);
         inputText.value = "";
         bindEvents(newListItem, crossTask);
        }
});